Alpha titanium alloys containing



Patented Aug. 11, 1959 ALPHA TITANIUM ALLOYS CONTAINING ALUMINUM AND COLUMBIUM 4 Claims. '(Cl. 75- -1755) N Drawing.

The invention relates to titanium base alloys containing only the alpha phase below 825 C. and more particularly to alloys of titanium containing aluminum and colurnbium; and this application is a continuation in part of our application Serial No. 372,161, filed August 3, 1953, now Patent No. 2,777,768, granted January 15, 1957.

Titanium metal transforms from a body-centered cubic or beta crystal structure or phase to a hexagonal closepacked or alpha crystal structure or phase on cooling from above 882 C. to below that temperature. An alloying element may be added to titanium to stabilize either the alpha or the beta structure. The effect of such an alloying element depends on whether the atomic diameter of the alloying element best fits into the hexagonal close-packed structure or the body-centered cubic lattice structure.

The effect of such an alloying element may be to either raise or lower the transformation temperature and there are known alpha stabilizers and beta stabilizers.

Alloying elements also may be added to titanium to develop some particular desired property or. properties and the addition of such alloying elements may affect the alpha or beta crystal structure of titanium indifferent ways.

For instance, there are many alloying elements that can be added to titanium as strengtheners but many of such elements cause a lowering of the transformation temperature which may be undesirable. In other words, the addition of one or more alloyingelements or metals to titanium for one purpose may adversely affect certain properties or characteristics that are desired to be present or developed in the alloy or in products formed therefrom. i

Aluminum, oxygen and nitrogen stabilize the alpha structure, and increasing amounts of these elements raise the transformation temperature. Below 825 C. aluminum is soluble in alpha titanium up to 24% by weight before the formation of a second phase (probably T iAl) begins. Similarly nitrogen and oxygen have solubilities of and 14%, respectively, in alpha titanium below 825 C. With these three elements, the amount which can be employed in titanium alloys, while still maintaining an all alpha structure below 825 C., is far greater than the amount which would'be desired in titanium base alloys from other considerations.

Columbium and vanadium, among others, are beta stabilizers, that is, their atomic size is such that they fit more satisfactorily into the body-centered cubic lattice structure, thereby lowering the transformationtemperature with increasing additions. However, there is a restricted alpha field with these alloying elements which makes possible the addition of very small amounts of these elements while preserving an 'all alpha structure below 825 C. Columbium and vanadium have solubilities with alpha titanium at 825 C. of approximately 1% and 1.5%, respectively.

When carbon, silicon or tungsten are added to tita' nium, there is a restricted alpha field; but instead of an alpha-beta field to the right of the all alpha region in the equilibrium diagram, there is an alpha plus compound region. In the case of carbon, this compound is titanium carbide. Although only 0.3% carbon is soluble in alpha titanium at 825 C. (the solubility decreases to0.2% at 600 C.), additions of greater. than 0.3-% carbon do not change the basic alpha structure but merely cause the presence of titanium carbide particles in the alpha matrix. Carbon can be intentionally added to titanium alloys, if desired, and it imparts useful physical properties up to 0.6%, notably increasing the tensile strength with a corresponding drop in ductility. Beyond this carbon content, ductility drops off rapidly.

Silicon is soluble in alpha titanium to an extent of 0.5% at 825 C. and any amount in excess of this, forms the compound Ti Si in the alpha matrix. High ductility titanium alloys have been made containing as much as 1.5% silicon, and having a considerable amount of in termetallic compound in the alpha matrix.

Tungsten does not form. a compound as do carbon andsilicon, and a tungsten content of greater than 0.4% at 825 C. results in a mixed alpha-beta structure. Below 725 C., the alpha-beta region changes to an alpha plus tungsten region but, because of the alpha-beta region present from 725 C. to the transformation temperature of 882 C., no more than 0.4% tungsten could be tolerated in a structure intended to be completely alpha.

These considerations present a rather complex problem in connection with pro iding a titanium base alloy in light gauge sheet form for the fabrication of products or parts from such titanium alloy sheets. In order to pro: vide such titanium alloy sheets, the sheets must be rolled and the required hot rolling temperatures for producing such sheets are in the neighborhood of, 800 C. The rolling proceduremay comprise hot rolling or may. include both hot and cold rolling to gauge, followed by an annealing operation so that a ductile titanium alloy sheet is produced which may be fabricated by forming or drawing to provide a desired part or end product. I

Accordingly, it is a fundamental, object of the present invention to provide a titanium alloy which does not go through a transformation on heating to and cooling from working or hot rolling temperatures in rolling sheets from such titanium alloy.

For example, difficulties have been encountered in hot rolling titanium alloys 'in'to sheets of desired gauge without harmful embrittlement where certain alloying elements are used for providing certain desired physical properties in the end product.

Also, the addition of certain alloying elements to titanium to provide desiredfphysical properties may adversely affect the welding characteristics of the alloy. Moreover, certain titanium alloys having desired physical properties at low temperatures lose strength rapidly and become brittle at high temperatures.

Another object of the present invention is to provide titanium alloys which may be hot rolled into sheets of any desired gauge without harmful embrittlement which may occur in hot rolling two phase titanium alloy sheets. This embrittlement in alpha-beta type titanium alloys, in which ductility cannot be restored by annealing, is believed to be caused by a brittle transformation product in the microstructure formed at relatively low temperatures by rapid cooling between the rolls during the hot rolling of thin sheets.

This transformation product is such that the transformation cannot be controlled during hot rolling and, because of this, the sheets cannot be annealed to form a consistently ductile material.

' There can be no embrittlement caused by a transformation of the microstructure during hot rolling, if no transformation occurs and no transformation products are formed during hot rolling. From the standpoint of hot rolling titanium alloy sheets, it is desirable, if not necessary, to heat the sheets being hot rolled to a rolling temperature of about 800 C. Thus, if no transformation occurs on heating a titanium alloy to or cooling it from 800 C., no embrittlement during hot rolling at such temperature can occur.

Accordingly, it is a further object of the present invention to provide a titanium alloy containing only the alpha phase below, say 825 C., thus raising the transformation temperature so that the crystal structure of the alloy does not change from alpha to beta and back from beta to alpha in heating the metal up to and rolling the metal at a rolling temperature of 800 C., and permitting such alloy to be satisfactorily hot rolled at 800 C.

More particularly, it is an object of the present invention to maintain an alpha structure in a titanium alloy which is stable at hot rolling temperature, and during hot rolling and cooling, so as to eliminate the occurrence of embrittlement during hot rolling such alloy into sheets of desired gauge.

It is a further object of the present invention to improve the welding characteristics of titanium alloys. We believe that for the same reasons discussed concerning embrittlement during hot rolling, welds of low ductility result when welding alpha-beta type titanium alloys even though high strength may be present in such welds. Many alloying elements can be added to titanium to strengthen the same, but unfortunately many of such elements lower the transformation temperature and provide low ductility. We have discovered that by raising the transformation temperature and by narrowing the alpha-beta field-that is the temperature range through which both alpha and beta can exist--to be as small as possible, there is an absence of any acicular transformation product in the alpha titanium alloy, providing ductile and high strength welds.

Moreover, it is an object of the present invention to provide an alpha titanium alloy having improved high temperature physical properties over titanium alloys containing metastable beta or mixed alpha and beta, which may lose strength rapidly and become brittle at temperatures above 300 C. In accordance with the present invention, the alpha titanium alloys comprehended are stable up to 825 C. in the annealed condition.

Furthermore, it is an object of the present invention to provide an alpha titanium alloy characterized by the absence of beta or the transformed beta phase below 825 C. which results in the elimination of brittleness obtained in hot rolled annealed titanium alloy sheets containing both the alpha and beta phases and the alpha and metastable beta phases, or the alpha and metastable beta phases. I

Moreover, it is an object of the present invention to provide an alpha titanium alloy containing no transformed or retained beta phase after having been hot rolled to 0.040 sheet material at 800 C.

The titanium base metal used in making the alpha titanium alloys of the present invention may be high purity titanium metal or commercial titanium as normally produced. In either event, the titanium base metal may contain substances or impurities normally found in either high purity titanium or commercial pure titanium such as carbon, oxygen and nitrogen in amounts varying with the degree of purity.

Sometimes one or more of the elements, carbon, oxygen or nitrogen, may intentionally be introduced in the alpha titanium alloys of the present invention. For example, all of the alpha stabilizers are potent strengtheners in connection with carbon, oxygen and nitrogen, but they are not so potent when carbon, oxygen and nitrogen are absent. Thus, as a general rule, a greater amount of one or more alloying elements may be necessary to develop certain properties or characteristics it carbon, oxygen and nitrogen are completely absent or only present in very small quantities.

Where prepared from commercial titanium, a typical analysis of the material in addition to titanium, aluminum and columbium is 0.02% C, 0.01% N 0.10% O and 0.005% H However, the invention is not restricted to use of material having the typical interstitial level indicated, as the level may be of the order of 0.15% C, 0.07% N 0.20% 0 and 0.025% H In other words, presently available sponge having a sponge hardness of 120 BHN is suitable. The sponge .hardness' may range from BHN to 200 BHN.

We have unexpectedly discovered a most interesting and significant characteristic regarding the alpha titanium alloys comprehended by the present invention, that the total amounts soluble in the alpha phase of two or more elements, can be included in one titanium alloy which will still be all alpha. This is important because the strength level of the resulting alloy depends on the amount of alloying agents added, and a Wide range of tensile strengths may be obtained, depending on the amount of amounts of alloying elements present. For instance, if 1.5% of X element, and 0.5% of Y element, and 3% of Z element each are soluble in the alpha phase, a total of 5% of the three alloying elements can be introduced into the alloy and the alloy will still be all alpha.

The alloys of the present invention may be manufactured by melting in an electric arc furnace in an atmosphere of argon or in a vacuum using a water cooled copper crucible to form ingots. The ingots may be scapled and then forged at suitable temperature to form slabs, the slabs being hot rolled to 0.040 sheet and then annealed, or hot rolled, annealed, then cold rolled and annealed to desired gauge. A typical annealing operation of 0.040 sheets rolled from alpha titanium alloys of the present invention may be carried out in a muflle type air furnace at 700 C. for one hour with air cooling. Annealing of alloys of the present invention in semi-finished or finished form, other than sheets of the stated thickness, may be carried out in the manner just described.

Typical examples of alloys of the present invention which have an all-alpha structure below 825 C. even though including columbium heretofore believed to be a beta stabilizer are a titanium alloy including 2% Al and 1% Cb; an alloy containing 2% Al, 1% V and 1% Cb, and an alloy containing 2% Al, 1% Cb and 1% Si.

Although certain percentages and combination percentages of the alpha titanium alloys containing aluminum and columbium of the present invention have been indicated in the examples, aluminum may be present in amounts of from 0.50% to 5.0%; vanadium may be present in amounts of 0.50% to 3.0%; silicon, from 0.05% to 1.5% and columbium from 0.50% to 1%, 1% being the approximate maximum amount of columbium soluble with alpha titanium at 825 C.

The maximum for vanadium, as indicated is 3.0% in making an alpha alloy since up to such an amount with the other alloying elements, vanadium has a good and not a detrimental efiect on the alloy as a whole. Vanadium has a tendency to hold the elongation at a certain level and obtain increased strength. The percentage of silicon can be on the high side if the percentages of aluminum and vanadium are on the low side.

.The alpha titanium alloys of the present invention incorporate in combination a number of desirable and outstanding characteristics and properties heretofore not found in combination in a titanium alloy. Thus sheets hot rolled from such alloys have a good combination of strength and ductility, providing as an average a 115,000 p.s.i. yield strength, with approximately 15% elongation in 2" as hot rolled or as hot rolled and annealed; along with a 3T minimum bend in sheets from 0.015"

to 0.125"; and provide weldability and lack of embrittlement at elevated temperatures up to 700 C. for four hours. In addition to the foregoing, the alloys possess a workable degree of reproducibility so as to be suitable for production if proper specification requirements of the sponge raw material are met.

Another matter worthy of comment is the fact that aluminum and silicon are good addition elements from the standpoint of lightness since they do not tend to increase the density of the resulting alloy.

Oxygen and nitrogen have been included in the fore going discussion, because although normally present in small amounts as impurities in titanium alloys, they tend to stabilize the hexagonal alpha structure and may be intentionally added for improvement of physical properties.

Accordingly, the present invention provides new alpha titanium alloys having outstanding new characteristics and avoiding difficulties, defects and undesirable characteristics heretofore present in unalloyed titanium sheets or titanium alloy sheets, the examples given being typical of the alloys comprehended by the present invention.

We claim:

1. An alpha titanium alloy consisting essentially of about: 0.5% to 5.0% aluminum, 0.5% to 3.0% vanadium, 0.5 to 1% columbium, balance substantially titanium with carbon, nitrogen and oxygen as incidental impurities, further characterized in that the aluminum, vanadium and columbium contents have solubility with alpha titanium at 825 C. and below.

2. An alpha titanitm alloy consisting essentially of about: 0.5% to 5.0% aluminum, 0.5% to 1% columbium, 0.05% to 1.5% silicon, balance substantially titanium with carbon, nitrogen and oxygen as incidental impurities, further characterized in that the aluminum, columbium and silicon contents have solubility with alpha titanium at 825 C. and below.

3. An alpha titanium alloy consisting essentially of about: 2% aluminum, 1% vanadium, 1% columbium, and the balance substantially titanium with carbon, nitrogen and oxygen as incidental impurities.

4. An alpha titanium alloy consisting essentially of about: 2% aluminum, 1% columbium, 1% silicon, and the balance substantially titanium with carbon, nitrogen and oxygen as incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS 2,754,203 Vordahl July 10, 1956 2,754,204 .Tafiee et al. July 10, 1956 2,777,768 Busch et a1. Jan. 15, 1957 FOREIGN PATENTS 1,094,616 France Dec. 8, 1954 

1. AN ALPHA TITANIUM ALLOY CONSISTING ESSENTIALLY OF ABOUT 0.5% TO 5.0% ALUMINUM, 0.5% TO 3.0% VANADIUM, 0.5% TO 1% COLUMBIUM, BALANCE SUBSTANTIALLY TITANIUM WITH CARBON, NITROGEN AND OXYGEN AS INCIDENTAL IMPURITIES, FURTHER CHARACTERIZED IN THAT THE ALUMINUM, VANADIUM AND COLUMBIUM CONTENTS HAVE SOLUBILITY WITH ALPHA TITANIUM AT 825*C. AND BELOW.
 2. AN ALPHA TITANIUM ALLOY CONSISTING ESSENTIALLY OF ABOUT: 0.5% TO 5.0% ALUMINUM, 0.5% TO 1% COLUMBIUM, 0.05% TO 1.5% SILICON, BALANCE SUBSTANTIALLY TITANIUM WITH CARBON, NITROGEN AND OXYGEN AS INCIDENTAL IMPURITIES, FURTHER CHARACTERIZED IN THAT THE ALUMINUM, COLUMBIUM AND SILICON CONTENTS HAVE SOLUBILITY WITH ALPHA TITANIUM AT 825*C. AND BELOW. 